How do you winterize an outside sprinkler system? Is it necessary in the northwest of Washington state?

Go to the Weather Channel website, type in the Zip and search weather history. It will give you winter temp ranges. Some systems have an auto drain do no water stays in the system. We don't know what system you have and how it was installed.

No question about it. You should winterize your system As Brian pointed out, some systems are install with slope so they can be drained for cold weather.
If yours isn't you could blow the water out with a air compressor or air tank. Good luck, Tom